Along with a decrease in the weight and size of recent electronic devices, the development of lithium-ion secondary batteries having high energy density has been advanced. In addition, as the lithium-ion secondary batteries become applicable to a wider range of fields, there is an increasing demand for improved battery characteristics. In particular, in the case of in-vehicle use of the lithium-ion secondary batteries, the safety and battery characteristics will be more important in the future.
Unfortunately, the lithium-ion secondary batteries are insufficient in safety in the cases where, for example, the batteries are overcharged, internally short-circuited, and penetrated by a nail. Thus, the safety of the batteries must be higher in the case of in-vehicle use. Further, in the case of in-vehicle use, the voltage is required to be higher than that currently used in order to increase the capacity.
As a method for improving the safety and increasing the voltage of an electrolyte secondary battery, use of a fluorine-containing ether having a specific structure has been proposed (for example, see Patent Literature 1). However, in the case of the electrolyte secondary battery of Patent Literature 1, the discharge capacity unfortunately decreases when the battery is left in a high temperature environment or when the battery is repeatedly charged and discharged.
As a method for improving the cycle characteristics of the lithium-ion secondary battery, it has been proposed to set the amount of alcohol in an electrolyte solution to less than 50 ppm (for example, see Patent Literature 2). Patent Literature 2 discloses the following: HF is generated as a result of a gradual reaction at room temperature between a fluorine-containing electrolyte solution and a diol or monoalcohol contained in a high-permittivity solvent such as ethylene carbonate or in a low viscosity solvent such as dimethyl carbonate; and such a reaction causes an increase in the amount of HF in the electrolyte solution along with the time, leading to a decrease in the battery cycle characteristics.
In addition, in order to provide a highly stable electrolyte solution having excellent withstand voltage and charge-discharge cycle characteristics as well as a high flash point, there has been proposed an electrolyte solution having an carbonate ester represented by formula [I]
wherein R1 represents an alkyl group or halogenated alkyl group; and R2 represents an alkyl group or halogenated alkyl group not having a hydrogen atom at the position β (for example, see Patent Literature 3).
In addition, in order to provide an electrolyte solution having excellent safety, high conductivity, and low viscosity, and to provide an electrolyte secondary battery containing the electrolyte solution, there has been proposed an electrolyte solution containing [A] and [B], the [A] being a solvent containing a fluorinated carbonate, a cyclic carbonate, and a chain carbonate, wherein (i) the cyclic carbonate content is 2 to 63 mol %, (ii) the chain carbonate content is 2 to 63 mol %, and (iii) the fluorinated carbonate content is 60 to 96 mol % (provided that the total amount of (i) to (iii) does not exceed 100 mol %), and the fluorinated carbonate is a compound represented by the following formula [1]:
wherein R1 and R2 may be the same or different; one of them is a C1-C4 hydrocarbon group in which at least one hydrogen atom is replaced by a fluorine atom; the other one is a C1-C4 hydrocarbon group or a C1-C4 hydrocarbon group in which at least one hydrogen atom is replaced by a fluorine atom; and these hydrocarbon groups include a group containing a heteroatom such as oxygen and nitrogen; and[B] an electrolyte (for example, see Patent Literature 4).